An in-mold foaming molded product which is obtained by (i) introducing polypropylene resin expanded particles into a mold and (ii) thermally molding the expanded particles with steam has characteristics such as freedom of shape, lightness in weight, and heat insulation efficiency which are advantages of an in-mold foaming molded product. An in-mold foaming molded product prepared from polypropylene resin expanded particles is compared with those prepared from expanded particles of similar synthetic resins as follows: An in-mold foaming molded product prepared from polypropylene resin expanded particles is superior to that prepared from polystyrene resin expanded particles in chemical resistance, heat resistance, and a distortion recovery rate after compression. In addition, an in-mold foaming molded product prepared from polypropylene resin expanded particles is superior to that prepared from polyethylene resin expanded particles in dimensional accuracy, heat resistance, and compressive strength. Because of those characteristics, an in-mold foaming molded product prepared from polypropylene resin expanded particles has various applications such as heat insulators, buffer packaging materials, automobile interior part materials, and core material for automobile bumpers.
However, in order to prepare an in-mold foaming molded product by fusing polypropylene resin expanded particles with each other in a mold, it is necessary to heat the polypropylene resin expanded particles at a high temperature, that is, under a high steam pressure in comparison with a case involving polystyrene resin expanded particles or polyethylene resin expanded particles. Therefore, the preparation of an in-mold foaming molded product is problematic in that it requires (i) a mold and a molding machine both tolerable against a high pressure and (ii) high steam cost for molding.
Most molding machines for in-mold foaming molding of polypropylene resin expanded particles have an upper limit of pressure resistance of approximately 0.4 MPa·G (gage pressure). Polypropylene resin expanded particles to be used for in-mold foaming molding are prepared from a resin which can be molded under the pressure up to the upper limit of the pressure resistance of molding machines. Generally, polypropylene resin expanded particles made of a propylene random copolymer having a melting point of approximately 140° C. to 150° C. are used.
However, due to a recent steep rise in fuel price etc., there is a demand for in-mold foaming molding performed at a lower temperature, that is, under a lower mold heating steam pressure. Further, in a case where an in-mold foaming molded product is prepared with use of, for example, a mold having a complicated shape or a large mold, there is a risk that expanded particles are not fused well to each other in some parts during molding. If the mold heating steam pressure is increased in order to perform the fusing sufficiently in such parts, the in-mold foaming molded product tend to deform or shrink. Accordingly, it is desired that a range of mold heating steam pressure (which may be hereinafter referred to also as “range of heating conditions for molding”) be wider than conventional.
A mainstream method for producing polypropylene resin expanded particles is a method of (i) dispersing polypropylene resin particles in water in a pressure-resistant vessel together with, for example, a dispersant in the presence of a foaming agent, (ii) heating the polypropylene resin particles under pressure to a predetermined foaming temperature, (iii) impregnating the polypropylene resin particles with the foaming agent, and then (iv) releasing the polypropylene resin particles to a low-pressure area for foaming.
Half-melting polypropylene resin particles in a pressure-resistant vessel at a foaming temperature, which is in the vicinity of a resin melting point, can produce expanded particles having two melting peaks during a differential scanning calorimeter (which may be hereinafter abbreviated to “DSC”) measurement of the expanded particles. Consequently, in-mold moldability of non-crosslinked polypropylene resin expanded particles is improved. A change occurring at that stage in the foaming temperature in the pressure-resistant vessel changes a DSC ratio [proportion of a high-temperature peak heat quantity to the total fusing heat quantity; QH/(QH+QL)×100(%) described below] of expanded particles having two melting peaks. This has problematically made it difficult to produce expanded particles having stable moldability.
In order to solve the problem of a high mold heating steam pressure, there have been proposed, each as a resin which has a low melting point and which has rather high rigidity in consideration of its melting point, (i) a propylene.1-butene random copolymer or a propylene.ethylene.1-butene random ternary copolymer, each prepared with use of a Ziegler polymerization catalyst (see Patent Literatures 1 and 2) and (ii) a PP homopolymer or a propylene.ethylene random copolymer, each prepared with use of a metallocene polymerization catalyst.
However, a propylene random copolymer containing a 1-butene comonomer, which is prepared with use of a Ziegler polymerization catalyst, has a limit in lowering its melting point. Specifically, melting points of commercially available propylene random copolymers are approximately 130° C. at the lowest.
On the other hand, a propylene.ethylene random copolymer, which is prepared with use of a metallocene polymerization catalyst, can have a lower melting point, specifically 130° C. or lower.
In order to perform in-mold foaming molding at a low heating temperature, there have been proposed polypropylene resin expanded particles made of a polypropylene resin having a resin melting point of 115° C. to 135° C. and an Olsen flexural modulus of 500 MPa or higher (see Patent Literature 3).
The resin used in Patent Literature 3 is, however, prepared partially from a propylene.ethylene.1-butene random ternary copolymer and mainly from a propylene.ethylene random copolymer prepared with use of a metallocene polymerization catalyst. Melting points of the resin in Examples in the specification of Patent Literature 3 are in a range of 120° C. to 134° C. Thus, although the polypropylene resin used in Patent Literature 3 enables in-mold foaming molding at a low heating temperature, the polypropylene resin used in Patent Literature 3 should be improved in terms of a range of mold heating steam pressure allowing use of a high temperature.
As a technique for widening the range of mold heating steam pressure, there have been propose polypropylene resin pre-expanded particles made from a mixture of two kinds of polypropylene resins of which a temperature difference between respective resin melting points is not smaller than 15° C. and not larger than 30° C. (see Patent Literature 4).
However, Patent Literature 4 requires a mold heating temperature of not lower than 140° C., and thus does not allow molding to be performed at a low temperature.
There have also been proposed similar techniques. Specifically, there have been proposed, in order to increase an peak temperature difference between a melting peak on a higher temperature side and a melting peak on a lower temperature side on a DSC curve for polypropylene resin expanded particles, (i) annealing polypropylene resin particles in a pressure-resistant vessel, (ii) using a blend of two or more kinds of polypropylene resins having respective resin melting points that are not much different from one another, the blend having a large difference between its resin melting point and its melting ending temperature, and (iii) blending two or more kinds of polypropylene resins having respective resin melting points that are greatly different from one another (see Patent Literature 5).
Patent Literature 5, however, discloses a lowest mold heating pressure of 0.20 MPa(G) (about 135° C.), and thus does not allow molding to be performed at a very low temperature.
Further, there have been proposed polypropylene resin expanded particles having a crystal structure which indicate (i) a main endothermic peak that exhibits an endothermic peak heat quantity of 70% to 95% of the total peak heat quantity and that has an apex temperature of 100° C. to 140° C. and (ii) two or more endothermic peaks on the higher temperature side of the main endothermic peak on the first DSC curve obtained when the polypropylene resin expanded particles are heated from normal temperature to 200° C. at a heating rate of 2° C./min by heat flux differential scanning calorimetry (see Patent Literature 6).
The polypropylene resin used in Patent Literature 6 is made from a mixture of (i) a propylene.ethylene random copolymer having a low melting point and prepared with use of a metallocene polymerization catalyst and (ii) a resin having a high melting point such as a propylene homopolymer prepared with use of a Ziegler polymerization catalyst. Examples in the specification of Patent Literature 6 describe that a satisfactory in-mold foaming molded product can be prepared with a heating steam pressure lower than 0.2 MPa·G, which pressure is lower than those used to prepare conventional polypropylene resin expanded particles.
However, according to the technique of Patent Literature 6, two component resins have an excessively large melting point difference therebetween, and therefore the expanded particles are expected to have a disturbed cell structure and to likely have an open-cell foam.